XMM-Newton observation of the highly magnetized accreting pulsar Swift J045106.8-694803: evidence of a hot thermal excess

Several persistent, low-luminosity (L-X similar to 10 (34) erg s(-1)), long-spin-period (P > 100 s) high-mass X-ray binaries have been reported with blackbody components with temperatures > 1 keV. These hot thermal excesses have correspondingly small emitting regions (< 1 km(2)) and are attributed to the neutron star polar caps. We present a recent XMM-Newton target of opportunity observation of the newest member of this class, Swift J045106.8-694803. The period was determined to be 168.5 +/- 0.2 s as of 2012 July 17 (MJD = 561 25.0). At L-X similar to 10 (36) erg s(-1), Swift J045106.8-694803 is the brightest member of this new class, as well as the one with the shortest period. The spectral analysis reveals for the first time the presence of a blackbody with temperature kT(BB) = 1.8(-0.3)(+0.2) keV and radius R-BB = 0.5 +/- 0.2 km. The pulsed fraction decreases with increasing energy and the ratio between the hard (> 2 keV) and soft (< 2 keV) light curves is anticorrelated with the pulse profile. Simulations of the spectrum suggest that this is caused by the pulsations of the blackbody being similar to pi out of phase with those of the power-law component. Using a simple model for emission from hot spots on the neutron star surface, we fit the pulse profile of the blackbody component to obtain an indication of the geometry of the system.